Absorbent nonwoven materials are used for wiping various types of spills and dirt in industrial, medical, office and household applications. They typically include a combination of thermoplastic polymers (synthetic fibres) and cellulosic pulp for absorbing both water and other hydrophilic substances, and hydrophobic substances (oils, fats). The nonwoven wipes of this type, in addition to having sufficient absorptive power, are at the same time strong, flexible and soft. They can be produced by wet-laying a pulp-containing mixture on a polymer web, followed by dewatering and hydroentangling to anchor the pulp onto the polymer and final drying. Absorbent nonwoven materials of this type and their production processes are disclosed in WO 2005/042819.
WO 99/22059 discloses a method of producing a nonwoven sheet material by providing melt-blown or spun-laid synthetic continuous filaments to form a polymer layer, applying a foam of natural (pulp) fibres on a side thereof through a head box to produce a combination of synthetic filaments and natural fibres, followed by hydroentangling the combination using water jets, to produce a composite sheet material in which the filaments and the natural fibres are intimately integrated resulting in high strength and high stiffness sheet material. The hydroentanglement can be preceded by applying the foam also on the other side of the polymer layer. WO 03/040469 teaches a similar process in which part of the starting materials is directly introduced into the head box, i.e. separate from the foam.
WO 2012/150902 discloses a method of producing a hydroentangled nonwoven material wherein a first fibrous web of synthetic staple fibres and natural (pulp) fibres is wet-laid and hydroentangled, spun-laid filaments are laid on top of the hydroentangled first fibrous web and a second fibrous web of natural fibres is wet-laid on top of the filaments and subsequently hydroentangled. The web is then reversed and subjected to a third hydroentangling treatment at the side of the first fibrous web, to produce a strong composite sheet material having essentially identical front and back sides.
Desirable results in terms of flexibility, sheet strength and absorption capacity are obtained when the pulp-containing web is produced by applying the pulp in the form of a foam containing a surfactant, onto or together with a synthetic polymer, and bonding the combined pulp fibres and synthetic polymer by hydroentanglement. However, surface irregularities or even thin spots or holes in the final sheet material may result, which negatively affect the sheet properties and performances as well as its appearance. This problem could be reduced by using relatively high levels of surfactant in the foam-forming pulp mixture, but high levels of surfactant turn out to hamper the hydroentangling process. In particular, it has been shown that high levels of surfactant may hamper the water purification in the recycling loop of water used in the hydroentangling, which in turn may interfere with the hydroentangling of the nonwoven material and hence result in suboptimum bonding in the nonwoven product.
Thus, there is a need for a process of producing hydroentangled nonwoven materials which avoids the drawbacks of irregular or defective surface characteristics and excessive use of surfactants.